1. Field of the Invention
The present invention relates to a heat roller type fixing unit in an image forming apparatus, and a control method for the same.
2. Description of the Related Art
In a heat roller type fixing unit in an image forming apparatus such as a copying machine, printer, facsimile apparatus, or the like, a cleaning web scheme is conventionally, generally employed as a cleaning means for removing toner remaining on the surface of a heat roller. This scheme is recognized as an effective technique because it can remove the remaining toner on the surface of the heat roller with a high efficiency and because it can be employed together with the function of applying a releasing agent such as silicone oil on the surface of the heat roller.
Where the cleaning web is in contact with the heat roller, however, the cleaning web is pulled by the frictional force or attracting force of the rotating heat roller. Sometimes the cleaning web is undesirably pulled out from the take-up shaft or feed shaft and reaches a transfer sheet feed path. Consequently, this scheme has a drawback in that inconveniences such as paper jamming may occur.
To prevent this, the feed shaft is braked in advance, so the web is prevented from being pulled out undesirably.
In this case, a driving force equal to or larger than the braking force of the feed shaft is required as a driving torque for the take-up shaft, and a very large tension acts on the cleaning web between the take-up shaft and the feed shaft. The tension of the web pulls a backup roller. Then, the backup roller disposed between the take-up and feed rollers acts in a direction to reduce the pressure on the heat roller. In particular, the axial center of the backup roller sometimes flexes undesirably in a direction to separate from the surface of the heat roller.
Generally, regarding the cleaning of the remaining toner with the cleaning web, the cleaning web which is conveyed between the backup roller and the heat roller while being pressed rubs against the heat roller with a predetermined pressure and nip width, so it cleans the remaining toner attaching to the heat roller.
When the backup roller and heat roller come into contact with each other, the resilient layer of the backup roller is depressed by the heat roller. A nip refers to a tight-contact region of the contact portion of the backup roller and heat roller. The tight-contact region usually has a rectangular shape (pattern) and changes into a drum-like shape or bobbin-like shape depending on the flexure of the shaft of the backup roller or the like. The nip width refers to the width of the nip in a direction parallel to the convey direction of the cleaning web.
When the backup roller flexes, the nip width enlarges at its two ends and narrows considerably at its center.
This leads to the following drawbacks. As the center of the backup roller has a small nip width and accordingly does not have a sufficiently large pressure, the remaining toner attaching to the heat roller does not attach to the cleaning web but may pass by under it. At the ends of the backup roller, the nip width is excessively large with respect to the take-up amount of the cleaning web, and accordingly that portion of the cleaning web where the remaining toner attaches cannot be taken up and recovered, but the toner which attaches to the cleaning web once may be discharged again.